In conventional stacks of electrochemical cells, the cells comprise a membrane electrode assembly sandwiched between bipolar plates. The plates usually act as the current collector and the electrode or ‘packing structures’ constitute the flow fields. It is necessary that the different elements of the cell are held together in the stack, and that pressure is applied. This is conventionally achieved by the use of tie rods around the periphery of the cell, arranged axially.
When the cell is sealed by the use of tie rods, it can sometimes be difficult to ensure that uniform pressure is applied to the whole active area (i.e. the membrane electrode assembly) of the cell. Another problem with this arrangement is that, while there is a good degree of pressure around the periphery of the cell, the centre of the cell, i.e. where the tie rods are not positioned, can sometimes bend outwards and lose pressure. Also, when the cells need to be serviced, the process of removing tie rods and numerous spring loading component is laborious.
Sometimes elastomeric elements are found compressed between the end plate and the cells at each extremity. The uniformity of pressure is better addressed by this type of cell but it is a permanent pressure that is applied. This is inflexible and uncontrollable other than by replacing of the part or member. This is a problem because the contact magnitude is central to ohmic losses between adjacent components, and therefore the overall efficiency.